It is known practice to produce (meth)acrylic esters, in particular C1-C4 esters, generally known as light (meth)acrylic esters or light (meth)acrylates, such as methyl acrylate or methacrylate, and ethyl acrylate or methacrylate, by direct esterification of (meth)acrylic acid with the corresponding alcohol, catalysed for example by sulphuric acid or an ion exchange resin.
The esterification reaction generates water and is generally accompanied by side reactions which produce impurities, in particular heavy compounds, that is to say compounds which have a high boiling point, higher than that of the desired ester.
In such processes, a final product of high purity is sought, while at the same time optimizing the starting material balance.
For these purposes, the light compounds with a boiling point below that of the ester—mainly the unreacted reagents—are isolated so as to be recycled to the reaction, and the heavy products generated during the process are separated before being exploited by treatment for example on a film evaporator and/or thermal cracking.
Consequently, a combination of treatments of the crude reaction mixture resulting from the esterification reaction is generally carried out, by means of a combination of distillations and/or extractions, and separations by settling out, which is both relatively complex to implement, and costly in terms of energy.
Schematically, the purification of the crude reaction mixture generally comprises at least the following steps:                the crude reaction mixture is subjected to a distillation in a first column, termed topping column, making it possible to obtain:                    at the top, a stream composed essentially of light compounds;            at the bottom, a stream comprising the desired ester, and heavy by-products;                        the bottom stream from the topping column is subjected to a second column, termed rectification column, making it possible to separate:                    at the top, the desired purified ester;            at the bottom, a stream containing essentially heavy by-products;                        the light compounds of the top stream from the first column are at least partly recycled, after optional separation, to the reaction;        the bottom stream from the second column is subjected to various treatments with a view to exploiting the heavy by-products.        
The use of these two columns in series has the drawback that the streams are subjected to high temperatures and to high residence times, in particular in the stripper associated with the first column, or at the bottom of the first column. The purified ester is obtained by distillation using the second column with elimination of heavy by-products. This heat exposure leads to the additional formation of heavy by-products by thermal 25 degradation, for example in the form of Michael adducts, thus reducing the purified-ester recovery yield and increasing the final amount of heavy impurities to be eliminated.
Moreover, the purification treatment needs to be controlled using stabilizers to limit the polymerization reactions that may occur under the operating conditions of the two columns. For this purpose, a first polymerization inhibitor, generally hydroquinone (HQ), is injected onto the first column, and a second polymerization inhibitor, generally methyl ether of hydroquinone (MEHQ), is injected onto the second column, resulting in a purified (meth)acrylic ester stabilized by methyl ether of hydroquinone which can be stored and transported before use.
A recent development in the distillation field has come to light under the name DWC (Divided Wall Column). This technology uses a single column, comprising an internal separating wall, which makes it possible to combine the operation of two columns that are conventionally in series, in a single piece of equipment, by using a stripper and a single condenser.
By way of example, patent application EP 2 659 943 describes a configuration of a divided wall column and the operation thereof in a process for producing 2-ethylhexyl acrylate of high purity. Although this column is complex to manufacture and to operate, it has the advantage of reducing the equipment cost and the energy consumption of the purification process, compared with a conventional facility comprising two distillation columns. The question of the stabilization required for it to function correctly is not, however, addressed. The purification process described in said document does not apply to the production of light acrylates by direct esterification. This is because, in the case for example of the synthesis of ethyl acrylate, the crude reaction medium comprises, in addition to the desired ethyl acrylate, also water produced by the reaction. Since ethyl acrylate and water have very close boiling points, separation of the water in a single column remains problematic.
Patent application JP 2005-239564 also describes the use of a divided wall column in a process for synthesizing (meth)acrylic esters, exemplified in the case of the synthesis of butyl methacrylate by transesterification reaction between methyl methacrylate and butanol. In this process, a demister is combined with the divided wall column so as to prevent the entrainment of stabilizer droplets in the sidestream withdrawal and to control the amount of stabilizers in the purified product. The divided wall column makes it possible to perform the separation of the targeted ester with the heavy products and the lighter products. The obtaining, by direct esterification of a light ester, such as ethyl acrylate with a boiling point close to that of water, corresponding to the commercial specifications is difficult to envisage in this process because of the presence of water that is difficult to eliminate using the divided wall column. It is recommended, in document JP 2005-239564, to eliminate the water beforehand using a solvent.
Document WO 2012/071158 describes a process for synthesizing C1-C4 alkyl (meth)acrylates, in particular butyl acrylate, in which the problem of the water arises during the esterification reaction step. Indeed, it is necessary to eliminate the water in order to shift the equilibrium of the reaction, but it is also necessary to keep the water in the reactor in order to promote certain azeotropes and to prevent the loss of ester and of alcohol at the top of the column. This is carried out by placing a decanter at the top of the reactor-distillation column assembly in order to separate an aqueous phase which is sent by reflux into the top of the column. The column surmounting the reactor may be a conventional column or a divided wall column, but it does not have the function of purifying the reaction mixture. In the process of document WO 2012/071158, the purified ester is obtained in a conventional purification section from the organic phase separated using the decanter placed at the top of the distillation column surmounting the reactor.
In the prior art documents, the additional problem of the separation of the water in a divided wall column for a process for producing light acrylates by direct esterification does not arise. Moreover, the presence of water in the medium to be treated requires more complex stabilization in order to limit polymerization reactions. It is necessary, on the one hand, to use a stabilizer for the organic medium and the aqueous medium that are present at the top of the column and, on the other hand, to stabilize the purified ester in order to obtain a “commercial” quality.
To the knowledge of the inventors, a purification technique combining a divided wall column and a decanter has never been used for purifying light (meth)acrylic esters, in particular ethyl acrylate, and the question of the stabilization required for it to operate correctly has not been addressed in the prior art.
The objective of the present invention is thus to provide a process for recovering a C1-C4 (meth)acrylic ester purified using a purification system comprising a divided wall column and a decanter.
The present invention thus provides a technico-economic solution to the problem of the purification of a crude reaction mixture resulting from the reaction of esterification of (meth)acrylic acid with a C1-C4 alcohol.